ISO/IEC have standardised video compression methods for standard resolution as well as high resolution video signals and pictures, the so-called MPEG-2 standard.
MPEG-2 provides for three different picture coding types: Intra (I), Predicted (P) and Bidirectionally predicted or interpolated (B) frames or fields. Still, in p and B frames or fields, macro blocks can be coded intraframe or intrafield, respectively, if motion prediction does not give good results. The decision has to be made in the encoder, with criteria to be determined by the implementation. In addition, MPEG-2 allows for frame or field based DCT (Discrete Cosine Transform), which requires sorting of lines and another decision to be made by the encoder.
In digital video broadcasting applications, decoders are usually needed in a high volume while only a small number of encoders are employed to provide the service. A typical configuration is given by the DirecTv satellite service in the USA, where MPEG-2 encoders for about 150 simultaneous programs provide a nation-wide broadcasting service. This requires relatively cheap decoders whereas encoders can include more exhaustive circuitry and can be more expensive.
MPEG-2 video encoding and decoding can be implemented in an asymmetrical manner in which encoders include a high degree of complex circuitry while decoders must include only that range of functionality absolutely required to decode valid bit streams.
For that purpose, MPEG defines the syntax and the semantics of the bit stream and a so-called System Target Decoder. The implementation of the encoder is not ruled by the standard. This allows one to built relatively simple encoders for applications with low demands on picture quality, as well as very complex encoders for high quality requirements.
For video broadcasting and entertainment applications reference levels for picture quality are set by existing video standards. For broadcasting of live events real-time operation of the encoder is a must. In order to cope with all different digital video applications and their respective requirements, the so-called MPEG Committee (Working Group 11 of ISO/IEC) has defined a set of Profiles and Levels. Profiles determine a certain subset of encoding tools belonging to the MPEG-2 standard. Different tools are used for specific encoding methods and applications. Levels restrict the vast ranges of parameters to those numbers which are used in common applications such as video broadcasting or video transmission over networks.
For video broadcasting services standard resolution video is related to ITU-R Recommendation 601 specifications or subsets of these, and can be handled by MPEG-2 Main Profile at Main Level (MP@ML). High definition video can be covered by the parameter sets defined in the High Levels of MPEG-2 (e.g. MP@HL).
The Main Profile comprises compression methods for video in the so-called 4:2:0 format, and it defines a variety of motion compensation and encoding modes, which are needed for high compression ratios. Motion compensation in the MP is based on frame and field based prediction in forward and backward direction, and includes refinements specifically targeted for interlaced video signals, such as the Dual Prime technique. For encoding, the MP allows frame and field based DCT, linear and non-linear quantization, standard and alternative zigzag scanning, and more.
Digital video broadcasting services will be based on bit rates in the range from as low as 1 Mbit/s up to about 10 Mbit/s. Raw data rates of video signals digitised according to the ITU-R Recommendation 601 comprise about 166 Mbit/s, without blanking intervals, clock and synchronisation. For a typical bit rate of 4 Mbit/s, which shall provide picture quality similar to existing NTSC and PAL video standards, a compression factor in the range of 40 is needed.
Compression ratios in this range are relatively easy to achieve for video signals which originate from film material, due to their non-interlaced nature and their low temporal repetition rate of 24 Hz. Much more demanding requirements come from real video signals originated from video cameras, especially in sequences with rapid and random motion. For adequate reproduction of such pictures at high compression ratios, all motion prediction and encoding alternatives provided by MPEG-2 MP are mandatory.
In particular, the standard does not specify at all, how video signals should be prepared for compression. In many cases, it is desirable to reduce the number of pixels before compression. The pixel decimation process can be applied in horizontal and vertical picture dimensions but in any case, it requires appropriate pre-filtering. The same applies to the conversion from the so-called 4:2:2 YC.sub.b C.sub.r format to the 4:2:0 definition, as required for MPEG encoding. Also, it is a well-known fact, that pictures with noisy behaviour are not well suited for compression, due to the random nature of the noise components. A variety of noise reduction techniques can be applied before encoding, for instance, field and frame recursive averaging methods. Furthermore, it is not necessary to encode fields which appear twice in the video signal due to 3:2 pull-down. In the pre-processing stage, a detection and elimination of redundant fields is useful and can be implemented in conjunction with other pre-processing methods. The same holds for horizontal and vertical blanking intervals in the video signal, which can be eliminated also before encoding, thus giving more processing time for the compression itself.